This is possibly the one I’ve most been looking forward to. It holds the record for amount of time elapsed between me creating a puzzle and actually publishing it. I made this almost  2 years ago, in the autumn of 2024, shortly after I had started making season 2 of Rat Run. I had recently decided that there would be 3 seasons and that season 3 would involve a 2nd rat. I had started making a list of good candiadates for “2 path” puzzles, such as zipper, palindrome and some other more experimental ones like mirror images. One evening I was watching a Zetamath stream and there was a duality puzzle being solved; a line with two different rules. It got me thinking about what would happen on a line which is both a parity line and a German Whisper. I’m sure this must have been done several times before by other people, but I hadn’t seen it. In typical ‘me’ fashion I started trying to draw the longest possible line I could. I wasn’t even thinking about Rat Run at the time. I realised such a line would be confined to only 4 digits, but I had two choices for which 4 digits. I also realised that I would need to somehow disambiguate the digits that weren’t on the line… so I thought about having two lines at once, which between them could include every digit except 5 , so could potentially disambiguate everything. In trying to draw the lines I noticed that they were so constrained that it was like I was trying to solve a puzzle myself. Whatever this was, it deserved to be a path drawing puzzle. With two paths rather than one. In a very satisfying light bulb moment, I suddenly thought of Phinx and realised this would HAVE to be an idea that I would use in season 3 of Rat Run. I excitedly got to work setting it straight away, even though I was busy making season 2 at the time, to a tight deadline of October 23rd. (I wanted to start season 2 on that day because it was exactly 10 weeks before Christmas Day, meaning I could have the Christmas Day finale for season 2 that I wanted.)

So, production of season 3 began a lot earlier than expected. At first it was tricky to find a layout for the two mega constrained paths without it breaking. I had a hunch that using symmetry might help me out… because of the high/low odd/even symmetrical nature of the two lines, I could vaguely imagine a layout with an organised pattern where all the digits slotted nicely in the gaps between each other, that would let me have two very long lines. This worked a treat… as soon as I started trying to draw the two lines symmetrically, I found a solution quickly (and this path layout was the one used in the finished version.) Having found a solution, I had a feeling it would only need a small number of clues to force the whole solvepath. I just needed to find the optimal choice and placement of clues. Redcurrants? Grapes? Forbidden doors?

I had some flexibility with the digits too… I could swap around some of the digit sets of my found solution and it would still be valid. Eg: on a line, as Simon realised, there are two monogamous digits (such as 4 and 7) and two polyamorous digits (such as 2 and 9.) so I could swap around 2s with 9s and 4s with 7s to give another perfectly valid line, and I could do the same on the other line (or not) and I could also swap both lines with eachother. So this gave me a bit more flexibility of clues I could place. Blackcurrants are usually a good choice for break-ins, and here was no exception. I realised that the identity of the lines could be determined quite early on using the blackcurrants, in a way that wasn't completely trivial. This exploited the fact that one line had two neighbouring blackcurrantable digits (1 and 8) whereas the other line (4927) was a lot less blackcurrant-friendly.

The layout of the paths were largely completely forced by sudoku geometry, with no other clues really needed. (In a similar way to episode 24 which I set around the same time - but even better!) I just needed a couple more very gentle clues to give a bit more information about the other line, and then magic took care of the rest. It doesn’t happen very often in setting but when it does it is the best feeling… with this set-up I had found a perfect sweet-spot between constrainedness and flexibility. Everything started filling itself in, and the path-finding was actually interesting and a perfect level of difficulty, yet it was not so constrained that it had been an absolute pain in the wotsits to set (like episodes 13, 31 and 37.). It used bishops colouring (one of my favourite techniques) and even had a natural deadly-pattern ending that didn’t need any deliberate engineering whatsoever. I had one extra one-way door in the puzzle in my first version (7>2 in the top right), but I later realised that even that wasn’t necessary, so I removed it. It was (and still is) one of my very favourite Rat Run episodes, and probably one of my favourite puzzles I’ve ever made in general. Minimalist and elegant puzzles along these lines are what got me hooked on CtC in the first place, but it's quite hard to set out to make those kind of puzzles - they are rare magical gifts that sometimes turn up once in a while. Although this was definitely more of a discovery than a construction, I’m still very pleased with my choice of 4 clues.

As soon as I'd set it, I knew that this episode would be positioned right near the end of season 3…  I knew I would probably have some other crazy rulesets for the second half of season 3, and I thought this one would contrast pleasingly with those, due to its nice short ruleset and bare grid, whilst still being a tough challenge worthy of its spot near the end. Felt like a nice climax in some ways, whilst being a return back to more simple beginnings in other ways, and thematically it ended up tying in well with the story too. I'm very pleased Simon found it tricky, because I genuinely thought this was a very hard puzzle too, and I published it at 5 stars. I thought maybe it might go down to 4, but I couldn't believe my eyes when it got voted down to 3!

It was not a difficult one to set though - it pretty much felt like it set itself. The hardest part was waiting a year and a half to show it to anyone!

I’m so pleased Simon enjoyed it and that people have finally got to see this thing at last :)

It's amazing how Marty just casually makes a puzzle that gets Simon pencil marking across boxes without even flinching. That says a lot about his brilliance! WOW!

36:26. And it’s not just Finkz and Phinx that have grown, because when this series started I wouldn’t have even tried to do this puzzle. ❤

Marty, you’re an absolute legend. I cannot believe that the paths ended up rotationally mirroring in about 3 different ways! 😯 The whole idea was both breathtaking and executed to perfection! 🌟👏❤

Quite an emotional rollercoaster for me.

1. Looked at Simon’s time - thought I could definitely attempt this one.
2. Stared at the empty grid for 5 minutes - almost abandoned the whole thing to get back to the video and watch the solve. Decided to at least try a few cells.
3. Started building a path from the bottom right corner, got lost at three cells & stared blankly at the grid for another 5 minutes.
4. Got the intuitive insight that the rat could never get to its “own” cupcake. Cheered.
5. Figured out that each path can in fact only have 4 distinct digits on it & disambiguating which is which should involve the black dots somehow. Cheered some more.
6. Flew through most of path building and filling the grid in no time, congratulated myself on being a genius.
7. Got hopelessly stuck at disambiguating the final path pieces in the end. Stared at the grid for another 15 minutes trying to figure out what technique to apply.
8. Gave up and bifurcated. Not proud of that one but there’s no way I could have figured it out analytically.

So happy I decided to go through with the solve, it’s an amazingly beautiful puzzle, if somewhat on the easy side once you break in.

i cant see the path he says when all cells on the path have been colored purple making it the clearest most easy to see path in rat run history

I have to thank you for publishing this solve late... over three days I kept coming back to this puzzle, unable to figure out the break in, frustrated that I couldn't get a hint here, until it finally clicked and I solved it without any help. Hooray!

The heat was bonkers today, I’m a trucker on night shift in UK. I had to sleep in my greenhouse of a truck all day, thankfully I have a 13 litre 530hp air conditioner built in 🥶

I hope Marty knows how happy the rat run series makes me. It always brings a smile to my face when I see the next video. They’re my favorite puzzles on the channel, always so entertaining and fun :)

At 1:03:10, the "ambiguity" about the 3s can immediately be resolved.  Both of those squares see r8c8, which is therefore a 5.  (The 3s and 8s up in Box 3 then rule out the other 3 in the "ambiguous" pair.)

There are various approaches to solving this puzzle.  Like Simon, I started with the table to find the two sets of four digits, and quickly realized they would work in symmetrical ways.  Unlike Simon, I focused first only on the path starting from the lower right, realized the second and third steps would be from the 1829 set (digits which connect to two different digits), and that the first and fourth steps would be from the 6347 set (monogamous digits).  I initially had two colors, one color for each of these sets, but immediately changed to four different colors, and that resulted in a very nice abstract coloring of the path with no regard to the other clues.  I had a yellow, green, blue, orange sequence for the lower rat, and then duplicated it with a pink, dark green, dark blue, dark red sequence for the upper rat.  There are five places on each path that have two choices, but otherwise, they were complete.  Only then did I use the four clues to finish the puzzle.  I think this approach was more elegant in the way it worked out.  One thing Simon explained that I did not fully grasp was the bishop coloring implications.  I particularly enjoyed the palindrome like oscillations of values that emerged from the rules as they interacted with the grid.

When I first saw the grid and rule set it seemed impossible for me, but once Simon played out the idea of the 4 digit lines, it suddenly sounded very intriguing.
I figured that the lines being made from a pair of monogamous digits and a pair of polygamous digits made them fully reversible and interchangeable. So I picked one orientation and started building a line from the bottom right, ignoring and other grid clues.
Both lines ended up reaching all the way to the cupcake with only a few options along the way. At that point I used the black dots in box 2 to disambiguate which line used which digits, then the inequalities to fully disambiguate the orders.

Martys puzzles have a way of making you feel so damn clever as the solver. Big props, good sir ❤❤

The first thing I did in this puzzle was to put a checkerboard coloring on the grid, which was quite helpful to solve it because along a given path there were only a couple of options for the number that could go into each color.

One of the few Rat Runs where I picked up the logic a bit quicker than Simon did, but I still had to stare at the puzzle a good while before I got there. Once I started seeing the path, I knew almost right away that it would be symmetrical, but I'm glad I was able to figure it out logically all the same. I actually managed to work on both paths simultaneously, though I still wound up with the same unknown corners that Simon did. Still, this was a very satisfying puzzle to solve despite the few clues and I can't wait to try the next one!

Fun fact that I haven't seen mentioned yet: Not only are the paths symmetrical, but Boxes 1, 5, and 9 are all identical, with only the 3 and 4 flipped in Box 5, creating yet another symmetry. Magic indeed! 😃

That's one of the best ones I've done in ages. I was actually talking out loud to myself because I was so impressed how smart it was. And I finally beat Simon at one! 🎉

So I got one of Marty’s t-shirts, the one with Rat Run 18.  Wore it once to show my daughter (a CTC fan). Second time to a party in Minneapolis, where the host’s roommate immediately yells “RAT RUN”!  I respond with “You’re my new best friend”!  Then we geeked out about CTC for about half an hour.

Hot, bothered, Guinness-soaked Simon is very amusing. From moments of self-flagellation for stupidity (unjustified), to moments of joy at discovery (justified), to moments of deep-seated admiration for Marty’s inimitable skills (very much justified), today’s video was most entertaining.

I do some competitive programming recreationally and there was something screaming at the back of my head that the puzzle will have a kind-of rotational symmetry.
That's a feeling I would follow during a contest. Possibly correct solution I can't prove is better than no solution at all.
I was right this time (9 goes always opposite to 8, 2 to 1, 5 to itself etc) but I hate that I can't articulate why it always had to be this way.

What fun, from start to finish. Thanks so much, Simon! (I will also say here that I am very glad that you prove things rigorously. Watching you do this has made me a better solver over the years. I have learned to be patient and really figure things out rather than jump to an intuition.)

I don't think I'll ever be ready, but alright Marty, bring on the 6 star Grand Finale that will leave us crying about the lack of Rat Runs in our future.