Dramatis Personae: King Edward III; The Countess of Salisbury; Lodowick.
Text & Context: Edward III was published anonymously in 1596, but is almost certainly from an earlier period. If not entirely by Shakespeare, it's fairly clear he had at least a hand in its composition. It contains a number of stylistic similarities to Shakespeare's early chronicle plays, and includes the line "lillies that fester smell far worse than weeds", which is the last line of Sonnet 94.
An interesting scene concerns Edward enlisting the aid of his secretary Lodowick to compose a love poem to the Countess of Salisbury. Edward becomes impatient with the suggested poetic clichés [cf. Sonnet 130] and takes over the writing of the poem himself. Explicit reference is made to Spenser's Shepard's Calendar: "then in the summer arbor sit by me, make it our council house, or cabinet: since green our thoughts, green be the conventicle, where we will ease us by disburd'ning them"; Spenser has his Colin Clout - "Colinet" - the keeper of Pan's green cabinet. Also mentioned is Apollo's spear, which both wounds and heals - a possible metaphor of poetry, mentioned again in Henry VI part 2. In the discussion of poetry between Edward and Lodowick, almost instantly invoked are Proserpina & Philomela, both rape victims as the Countess of Salisbury is about to become.
Not only did Edward III rape the Countess of Salibury, but he did so in a excessively violent manner over a series of days. The Order of the Garter with its pomp and ceremony and honi soit qui mal y pense ["evil on him who thinks evil of it"] was Edward's attempt at expiation for his deed. While this is not explicitly exposed in the play, Edward's un-kingly lust is, along with his eagerness to murder for the mere satisfaction of his libido - this, rather than some slightly unflattering portrayal of the Scots, is likely why the play was never revised by the author and so remained unincorporated into the larger saga of nationhood that comprise Shakespeare's chronicle plays.