candidate: Rule 14 (a) (2) : When properly served , the third - party defendant must assert any defense against the thirdparty plaintiff 's claim under Rule 12 ; must assert any counterclaims against the third - party plaintiff that are compulsory under Rule 13 (a) ; may assert any counterclaim against the third - party plaintiff under Rule 13 (b) or any crossclaim against another third - party defendant under Rule 13 (g) ; may assert against the plaintiff any defense that the third - party plaintiff has to the plaintiff 's claim ; and may also assert against the plaintiff any claim arising out of the transaction or occurrence that is the subject matter of the plaintiff 's claim against the third - party plaintiff .