An idiomatic expression is a phrase or group of words whose meaning cannot be deduced from the literal words used. In the sentence above, 'to take the rap', simply means to take the blame for an offense for which the individual may or may not be guilty of. So, Sheila was blamed or punished when her brother broke the antique lamp. The native speakers of a language are quite familiar with the idiomatic expressions in their language.
I knew that the phrase, 'took the rap', was an idiomatic expression because I could not easily deduce its meaning from just the literal words used. So I understood that there must have been a deeper meaning to it.
The sentence that states to help save the day, here he must get past a scary three headed Guard dog. This is because in the Greek Mythological literature there are lots of creatures such as three headed monster’s like the guard dog.
