Professor Farnsworth plans to destroy a yellow box containing a dangerous experiment by ejecting it into the sun.
They also discover that coin flips have opposite outcomes, which explains why the parallel Leela did not open the box she was guarding.
To ensure the safety of both, the Farnsworths exchange their universe-boxes by pulling each through the other, meaning that each now paradoxically has a box containing their own universe.
Fry asks Leela on a date; she flips a coin, then decides to accept without viewing the result.
[1] The 2013 revision of the list bumped the episode up to number 16, reassessing it as better than "The Devil's Hands Are Idle Playthings", "Space Pilot 3000", the two "Anthology of Interest" episodes, "Parasites Lost", "Fry and the Slurm Factory", and "Love's Labors Lost in Space", all of which had previously been ranked higher.