Hey there :)
As we need to find the subject, first, subject means a word that is the main head or important clause referring to the sentence.
Frankly, we can say that subject is the sentence phrase used to show the main highlighted word of a particular sentence or the diet of the action. All other actions are based on this subject.
So, as per question, Martin Manhunter is the subject because the complete sentence is occured due to Martin's bed.
