Cite as 508 F.2d 909 (1974)
in detail, it will be necessary to repeat some of the more salient factual background.
I
The issue presented is who has the copyright renewal rights in the famous “Superman” cartoon character. In 1933, plaintiff Jerome Siegel conceived the idea of Superman—a person of unprecedented physical prowess dedicated to acts of derring-do in the public interest. In the same year, together with plaintiff Joseph Shuster, he created a Superman comic strip which consisted of several weeks’ worth of material, some of which was completely “inked in” and ready for publication, and some of which consisted of mere black-and-white pencil drawings. The new strip was not published at this time, although plaintiffs continued to collaborate on other strips that were then in publication.
One of plaintiffs’ customers for these other strips was Detective Comics, Inc. (Detective), the predecessor in interest of the present defendant National Periodical Publications, Inc. (National). On December 4, 1937 Detective entered into a written employment contract with plaintiffs, whereby the latter would furnish two other strips exclusively to Detective for a period of two years. The plaintiffs further agreed “that all of these products and work done by said Employee [plaintiffs] for said Employer [Detective] during said period of employment, shall be and become the sole and exclusive property of the Employer, and the Employer shall be deemed the sole creator thereof, the Employee acting entirely as the Employer’s employee.” The agreement also gave Detective the right of first refusal for any new strips the plaintiffs might produce.
In 1938, for the first time, plaintiffs submitted their 1933 Superman materials to Detective to be considered for use in a new magazine, Action Comics. Detective asked plaintiffs to revise and expand the materials into a full-length production suitable for magazine publication, and this was done.
Thereafter, and prior to the first appearance of Superman in Action Comics, Detective prepared a written release, dated 1 March 1938, which was executed by plaintiffs and accepted by Detective. This release sold and transferred to Detective “such work and strip [Superman], all good will attached thereto and exclusive rights to the use of the characters and story, continuity and title of strip contained therein, to you [Detective] and your assigns to have and hold forever and to be your exclusive property. … The intent hereof is to give you exclusive right to use and acknowledge that you own said characters or story and the use thereof exclusively ….”
On 19 December 1939, a supplemental employment agreement was entered into. It raised plaintiffs’ compensation for the increasingly popular Superman strip, and in addition contained the following language:
That we, Detective Comics, Inc., are the sole and exclusive owners of the comic strip entitled “Superman” … and to [sic] all rights of reproduction of all said comic strips and the titles and characters contained therein, and the continuity thereof, including but not limited to the fields of magazine or other book publication, newspaper syndication … and all other form of reproduction. We have all right of copyright and all rights to secure copyright registration in respect of all such forms of reproduction. …
No further written agreements between the parties were ever executed, although plaintiffs were paid compensation at ever-increasing rates. By 1947, plaintiffs’ total compensation for the strip exceeded $400,000.
In that year plaintiffs instituted an action against defendant National in the New York State Supreme Court, Westchester County, to annul the contracts between the parties for various reasons. The March 1938 agreement was specifically attacked as void for lack of mutuality and consideration. In addition, plain-
