FBI, FBI: Most Wanted and FBI: International
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FBI 1x18 is the backdoor pilot to FBI: Most Wanted
FBI episode 2x18 and FBI: Most Wanted episode 1x09 are crossover episodes
FBI 2x19 is a character crossover with Chicago P.D. (7x18 is the introduction on P.D. Not a must watch, but you get background information)
FBI Order
by Yasve