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A law student, Julie Parlo, asks Monk to find her kidnapped grandmother. In exchange, she promises to use her legal knowledge to help him get reinstated to the police force. Watch Monk S02E 13 1
The kidnappers are surprisingly "affably evil," treating the grandmother well and even feeding her pizza while she is held. Where to Watch The episode you are looking for is titled
Monk eventually realizes the kidnappers weren't interested in the grandmother at all, but rather the chair she was tied to. Watch Monk S02E 13 1