What is the defence of the headnote in a promissory note case?

In McQuarrie v. Brand, 28 O.R. 69, the headnote is as follows: It is a good defence to an action by the personal representatives of the payee against the maker of a promissory note for value received, that at the time of the making of the note an oral agreement was entered into between the payee and the maker which has been fully performed, that if the latter would pay interest on the note, and, although not liable to do so, would support for life a relative of the former, the note should be considered paid; and evidence to the above effect was held admissible in an action on the note brought after the complete performance of the agreement by the defendant.