Strange Query Plan

I have a query like below .. if i add where Served = 1 , the query takes foreever... if i remove it, it takes only 6 sec...

I am not sure why this is hapening?


select distinct a.Episode_Key,
case when ag.Category IN ('ASMI', 'COOC', 'SPCL') then 'SMI'
when ag.Category = 'SEDC' then 'SED'
when ag.Category = 'ACCA' then 'SA'
when ag.Category like 'CGA%' then 'Gam'
end as [Category],
ag.Agreement_Type_Name as [Agreement],
p.ServiceProvider,
s2.Served
from dbo.Assessment a
INNER JOIN (
select distinct Episode_Key, p.ServiceProvider, max(CSDS_Object_Key) as [Sequence]
from dbo.Assessment a
INNER JOIN dbo.CD_Provider_Xref p
ON a.Provider_CD = p.Provider_CD
where Creation_DT >= '07/01/2007'
and Reason_CD = 1
group by Episode_Key, p.ServiceProvider
) as s1
ON a.CSDS_Object_Key = s1.Sequence
INNER JOIN dbo.CD_Provider_XREF p
ON a.Provider_CD = p.Provider_CD
INNER JOIN dbo.CD_Agreement_Type ag
ON ag.Agreement_Type_CD = a.Agreement_Type_CD
LEFT OUTER JOIN (
select distinct Episode_Key, p.ServiceProvider,
1 as [Served]
from dbo.Encounters e
INNER JOIN dbo.CD_Provider_Xref p
ON e.Provider_CD = p.Provider_CD
where Encounter_Begin_DT between '01/01/2008' and '01/31/2008'
and Procedure_CD is not null
and Encounter_Units > 0
) as s2
ON a.Episode_Key = s2.Episode_Key
and p.ServiceProvider = s2.ServiceProvider
????---where Served = 1
group by a.Episode_Key, ag.Agreement_Type_Name, p.ServiceProvider, Served,
case when ag.Category IN ('ASMI', 'COOC', 'SPCL') then 'SMI'
when ag.Category = 'SEDC' then 'SED'
when ag.Category = 'ACCA' then 'SA'
when ag.Category like 'CGA%' then 'Gam'
End