What story Cassius tells Brutus?

He tells Brutus a story in which he & Caesar were holding a swimming contest across the Tiber river, & Caesar starts to drown.

The story from the text is as follows::

For once, upon a raw and gusty day,
The troubled Tiber chafing with her shores,
Caesar said to me 'Darest thou, Cassius, now
Leap in with me into this angry flood,
And swim to yonder point?' Upon the Word,
Accoutred as I was, I plunged in
And bade him follow; so indeed he did.
The torrent roar'd, and we did buffet it
With lusty sinews, throwing it aside
And stemming it with hearts of controversy;
But ere we could arrive the point proposed,
Caesar cried 'Help me, Cassius, or I sink!'
I, as Aeneas, our great ancestor,
Did from the flames of Troy upon his shoulder
The old Anchises bear, so from the waves of Tiber
Did I the tired Caesar. And this man
Is now become a god, and Cassius is
A wretched creature and must bend his body,
If Caesar carelessly but nod on him.
He had a fever when he was in Spain,
And when the fit was on him, I did mark
How he did shake: 'tis true, this god did shake;
His coward lips did from their colour fly,
And that same eye whose bend doth awe the world
Did lose his lustre: I did hear him groan:
Ay, and that tongue of his that bade the Romans
Mark him and write his speeches in their books,
Alas, it cried 'Give me some drink, Titinius,'
As a sick girl. Ye gods, it doth amaze me
A man of such a feeble temper should
So get the start of the majestic world
And bear the palm alone.

Hope this helps!

--Salena Jane--