6. The Mole
(“Walter Taffet,” season 3, episode 7)

In an incredible callback, the bug Martha planted in FBI Agent Gaad’s (Richard Thomas) office in Season 1 comes back in a big way when it is discovered midway through Season 3. The man investigating the bug works for the FBI Office of Professional Responsibility, which is the office Clark (a.k.a. Philip’s cover persona in his relationship with Martha) has always claimed he works for. Martha’s entire world comes crashing down.

We also learn that Philip has a 20-year-old son from a pre-spy relationship, something that is going to come home to roost in the upcoming fifth season.