"Dan Vs. Chris" is the 17th episode of the second season.
After Chris wins a Death Ray Gun prop in a radio contest for the classic movie "Space Monkeys From Planet Space," Dan disputes Chris's right to the prize and swears revenge.
Dan goes through a small box of his keepsakes for revenge supplies, starting with a gas mask and a lucky hand grenade. His first attempt involves lowering a skunk down Chris and Elise's chimney and getting it to spray, driving the two out. This works, but when Dan goes into the house with the gas mask to take the Death Ray Gun, he finds it encased in glass. Bypassing the glass case takes too long, and Chris and Elise throw Dan out.
Later, Dan is driving his car when he hears Chris getting an interview for winning the Death Ray Gun. Angered and distracted, Dan crashes into an RV belonging to Elise's parents. They come out of the RV to confront him, but when Dan explains the situation, they are eager to help (because they don't like that Elise married Chris).
At first, Dan turns down their plan and their help due to stylistic differences. But when Dan returns to his apartment on foot, he finds Chris has given away nearly all of Dan's belongings; Chris justifies this flippantly by saying Dan clearly can't tell the difference between Chris's stuff and his own. Dan throws his grenade at Chris, but it doesn't detonate. Once Chris leaves, Dan mopes with a bowl of cereal, only to realize Chris has switched Dan's lactose-free milk with cream, leaving Dan with hours of crippling stomach pain. Once he can get to the phone, he tells Elise's parents he will join them on their plan.
The scheme is to set up a machine to put Chris in a box and ship him to a ball bearing factory in Siberia. They build the machine in Chris's garage. When Chris arrives and stands on the X, however, the machine fails because Dan's grenade is stuck in the gears, and this failure burns down the garage.
Chris wishes to finish this, so he challenges Dan to a battle to the death at Felix Sanches State Park. They battle each other with special monkey fist staffs with hidden electrode weapons. (Elise put one in Chris's staff, and her parents independently put one in Dan's staff.) The battle is tense, and Dan almost knocks Chris off a cliff and wins, but when he looks into Chris's eyes, he remembers all the good times they had. He helps Chris up and goes home.
At Dan's apartment, Chris gives Dan his Death Ray Gun and Dan's box of memories. All is forgiven, and they plan to watch "Space Monkeys From Planet Space" later. Then Dan goes to the bathroom and gets caught in the trap Elise's parents rebuilt there. Arriving in Siberia one week later, Dan bemoans his luck and not being at home in bed. Seconds later at his home, the "lucky" hand grenade explodes, destroying the box it was in, the Death Ray Gun it was with, and the mattress the box was under.
The only named character from SMfPS is Captain Jack Tatterson. He notably used the Death Ray Gun to defeat the Blorg atop Mount Climax, and this sent all the monkeys into the Greenscreen Abyss of Planet Space. This might be different from Tatterson's duel to the death with the Space Monkey Overlord.
"Climax" is a term for a part of a story near the end where the action peaks. "Greenscreen" is a method of compositing footage in film. These geographic names, plus the name Planet Space for the setting, suggest that the creators of SMfPS weren't very creative.
"Blorg" doesn't mean anything obvious, but sounds similar to both the Blob and the Borg.
The elementary school that answers the trivia contest question at the start of the episode, William Henry Harrison, is named for the ninth President of the U.S. This is an unlikely namesake for a school, due to Harrison dying of pneumonia 31 days after his inauguration. (Chris's misspoken but accepted answer, "William Harry Henderson," is mostly a meaningless transposal, but resembles the band name "Harry and the Hendersons.")