The Thing That Wouldn't Die is a two parter series finale of the show.
After Mary Albright witness Dick turn Liam into a monkey to stop him from turning Earth into Planet Monkey World, Dick made a confession to Mary that he's a alien from another planet. She didn't believe him at first, but after Dick told her to think back when they were together for the past 6 years, she believes him.
Meanwhile, Don Orville made a confession to Sally that he was a coward. He joined the police force so he can be safe from the bad guys. He decided to open up a muffin shop, to much of Sally's disgusted.
At the same time, Harry Solomon made a date with a girl named Samantha (AKA Sam), who he thought she has a twin sister due to being called Sam, or Samantha.
Unfortunately, The Big Giant Head called off the mission due to Dick's action, and have to leave Earth on Saturday at midnight. In response to this, Dick invited Mary to spend the rest of her life on their planet, and Sally reshape Don.
After a farewell party, Mary decided to stay on Earth. In which Dick agreed. After knocking her out, he gives a one last kiss before leaving. The episode ends with the Solomon sing their mission song, as they beam up to their spaceship.