hook, line and sinker
hook, line and sinker {adv. phr.}, {informal}
Without question or doubt; completely.
Johnny was so easily fooled that he fell for Joe's story, hook, line and sinker.
Mary was such a romantic girl that she swallowed the story Alice told her about her date, hook, line and sinker.
Bobby trusted Jim so he was taken in by his hard-luck story hook, line and sinker.
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Source: A Dictionary of American Idioms