Neither Mary nor Jamdes plan or plans to attend the convention.
I'm sorry to disagree with you, Anglika, but I thought the rules were:
Rule 1 Two singular subjects connected by or or nor require a singular verb.
Example My aunt or my uncle is arriving by train today.
Rule 2 Two singular subjects connected by either/or or neither/nor require a singular verb as in
Examples Neither John nor Susan is available.
Either Kiana or Casey helps today with stage decorations.
Rule 3 When one of your two subjects is I, put it second and follow it with the singular verb am.
Example Neither she nor I am going to the festival.
Rule 4 When a singular subject is connected by or or nor to a plural subject, put the plural subject last and use a plural verb.
Example The book or the magazines are on the shelf.
Rule 5 When a singular and plural subject are connected by either/or or neither/nor, put the plural subject last and use a plural verb.
Example Neither Jenny nor the others are available.
Subject and Verb Agreement | Grammar Rules